1. Technical Field
The present invention relates to apparatus and methods for analysis of blood samples in general, and apparatus and methods for detecting and enumerating platelets, and differentiating platelets from giant platelets, and giant platelets from platelet clumps, in particular.
2. Background Information
Physicians, veterinarians and scientists have examined human and animals' biologic fluids, especially blood, in order to determine constituent quantities as well as to identify the presence of unusual particulates not seen in healthy subjects. The constituents generally measured, quantified and identified include red blood cells (RBCs), white blood cells (WBCs), and platelets.
In mammals, platelets (also referred to as thrombocytes) are small irregularly shaped anuclear cell fragments that are derived from fragmentation of megakaryocytes. Thrombocytes in certain animals (e.g., birds, reptiles and fish) are similar in function to mammalian platelets, but are about ten times larger and nucleated. Platelet analyses can include the number, size, shape, texture, and volume determinations of the platelets within the sample, including the determination of the presence of clumps of platelets or thrombocytes within the sample. Under certain naturally occurring conditions, platelets will aggregate into clumps within a subject as a useful response to a trauma (e.g., hemorrhaging, tissue trauma, etc.) experienced by the body. Platelet clumps forming within a blood sample collected for analysis, on the other hand, are typically not useful and can hinder the analysis of the blood sample. Anticoagulants (e.g., EDTA) can be used to prevent platelets from clumping within a sample, but clumps may still form if there is delay in mixing the anticoagulant with the blood sample. Once clumps form, anticoagulants are typically ineffective in separating them into individual platelets. Platelet clumps are often problematic within a sample being analyzed because they can lead to erroneously low platelet counts, which can lead to misdiagnosis and serious consequences to the patient.
Known blood examination techniques, described in detail medical texts such as Wintrobe's Clinical Hematology 12th Edition, generally divide the examination methods into manual, centrifugal, and impedance type methods. Manual methods typically involve the creation of an accurately determined volume of a blood or fluid sample that is quantitatively diluted and visually counted in a counting chamber. Manual examination methods for cell enumeration include examining a peripheral smear where the relative amounts of the particulate types are determined by visual inspection. Centrifugal examination methods involve centrifuging the sample, causing the sample to separate into constituent layers according to the relative densities of the constituents. The component layers can be stained to enhance visibility or detection. Impedance methods involve the examination of an accurate volume of blood which is treated according to the particulate being measured; e.g., lysing RBCs for enumeration of the nucleated cells and volumetrically diluting the sample in a conductive fluid. The process typically involves monitoring a current or voltage applied to sample passing through a narrow passage to determine the effect particulates have on the current/voltage as the particulates pass through in single file. Other techniques involve analyzing the intensity and angle of scatter of light incident to particulates passing single file through a light beam. Flow cytometric methods can also be used that involve staining particulates of interest in suspension with fluorophores attached to antibodies directed against surface epitopes present on cell or particle types, exciting the stained particulates with light of appropriate wavelengths, and analyzing the emission of the individual particulates/cells.
All of the aforementioned methods, other than the peripheral smear or centrifugal separation, require dispensing a precise volume of sample. Inaccuracies in the sample volume will result in quantitative errors of the same magnitude in the associated analysis. With the exception of centrifugal methods, all of the aforementioned methods also require the sample to be mixed with one or more liquid reagents or diluents, and also require calibration of the instrument to obtain accurate results. In the case of peripheral smears, a high degree of training is needed to properly examine the smear. A number of the aforementioned methods generate large volumes of contaminated waste which is expensive to handle. Additionally, the above-described methods are not suitable to determine the complete blood count (CBC) in birds, reptiles and fish, where the red blood cells and thrombocytes are nucleated, and in certain mammals where the red blood cells size is very small and may be confused with platelets.